Japanese Patent Application No. 2002-26558, filed on Feb. 4, 2002, is hereby incorporated by reference in its entirety.
The present invention relates to an operational amplifier circuit, a drive circuit, and a method of controlling an operational amplifier circuit.
A liquid-crystal panel operated by a simple-matrix method and a liquid-crystal panel operated by an active-matrix method using switching elements such as thin-film transistors (hereinafter abbreviated to TFTs) are known in the art as liquid-crystal panels (electro-optical devices) for use in electronic appliances such as mobile phones.
The simple-matrix method has the advantage of a low power consumption in comparison with the active-matrix method, but it has a disadvantage in that it makes it difficult to display multiple colors and moving images. The active-matrix method, on the other hand, has the advantage of enabling multiple colors and moving-image displays, but has a disadvantage in that it is difficult to reduce the power consumption.
Demands for multiple colors and moving-image displays have recently increased, in order to provide high-quality images in portable electronic appliances such as mobile phones. For that reason, it is becoming common to use an active-matrix method liquid-crystal panel instead of the simple-matrix method liquid-crystal panel that has been used up to now.
With an active-matrix method liquid-crystal panel, it is preferable to provide an operational amplifier circuit (op-amp) that functions as an output buffer, in the data line drive circuit that drives the data lines of the display panel. In the conventional art, it has become common to use a class-AB (push-pull system) operational amplifier circuit as this operational amplifier circuit.
However, the conventional class-AB operational amplifier circuit has a defect in that there is a large number of paths along which the current flows, so there a lot of current is consumed wastefully and thus the power consumption is large. If an attempt is make to throttle the current to reduce power consumption, that could lead to a state in which the response speed drops and the frequency characteristic deteriorates.
An aspect of the present invention relates to an operational amplifier circuit comprising:
a differential section which amplifies a differential value of differential signals;
an output section including a first drive transistor of a first conductivity type and a second drive transistor of a second conductivity type, the first drive transistor being provided on a first power source side, the second drive transistor being provided in series with the first drive transistor and provided on a second power source side, and a gate electrode of the second drive transistor being controlled based on a voltage at an output node of the differential section;
a voltage setting circuit which sets a given first voltage to a first node connected to a gate electrode of the first drive transistor; and
a first capacitance element provided between an output node of the differential section and the first node,
wherein the voltage setting circuit sets the first voltage to the first node, and then the first capacitance element transfers a voltage change at the output node of the differential section to the first node.